As is known, DC powered devices may be destroyed from improper installation of a supply voltage. Improper installation of a supply voltage results when the supply voltage is at an excessively high potential or when the supply voltage is installed with reverse polarity. Protection from an improperly installed supply voltage is highly desirable to inhibit possible destruction of the DC powered device.
To protect against reverse polarity, prior art protection designs included blocking diodes. In such circuits, the diode conducts when the supply voltage is connected with proper polarity. When the supply voltage is connected with reverse polarity, the diode does not conduct. When the diode conducts, it contributes 0.3 to 1.0 voltage loss to the circuit. The voltage loss results in associated power loss to the circuit as well.
To protect from over voltage, one implementation uses a fuse and clamping zener diode. When an over voltage condition occurs, the zener diode conducts, clamping the supply and blowing the fuse. While this design offers protection, the protection elements, zener diode and/or fuse, are not reusable once a supply voltage has been incorrectly installed in the circuit. For example, when a supply voltage is improperly installed, the zener diode and/or fuse is destroyed rendering the DC powered device inoperable. The diode or fuse would have to be replaced before the DC powered device would be operable with a properly installed supply voltage. Depending on the location of the protection element in the circuit, replacement may not be an easy task.
Another implementation to protect against over voltage utilizes a single bipolar junction transistor, (BJT), connected in series with the supply. The BJT acts as a switch; when the supply voltage is below a certain level, the BJT is on; when the supply voltage is above the threshold, the BJT is off. When the BJT is on, it may produce unacceptable losses in certain applications. This is due to the amount of base current required to cause a BJT to conduct, i.e. be on. Examples of prior art protection circuits are the Motorola MC3399T and the Maxim MAX625.
Therefore a need exists for a means and a method for protecting DC powered devices from over and reverse voltage supplies while minimizing the drawbacks of the prior art solutions.